AC/DC converters typically require isolation between primary and secondary circuits. Only very specific components are allowed to bridge this boundary. Examples of such components are optocouplers, transformers, and high voltage Y-capacitors. AC port surge requirements place a large surge voltage across this boundary. One way to improve surge immunity is to place a physical slot in the circuit board at the boundary between leads or traces that are at greatest risk for arcing. Typically these slots are added between the leads of components bridging the isolation boundary.
The PC boards define slots between primary and secondary components to reduce, or prevent, arcing. As soon as an arc occurs the air ionizes and approaches a short circuit. For a typical 10 kV, 2 Ohm test, current on the order of 5000 Amps can flow during arcing. Arcing typically occurs between any primary side component, or trace, and any secondary side component, or trace. The closer the points, the more likely they are to arc. Geometry also has an impact. Arcing occurs more likely between two sharp points than two smooth points with the same spacing between them. In the field, arcing can occur as a result of a common mode AC power surge, or lightning.
A slot between primary and secondary helps because without the slot, contaminants, such as, for example, specs of solder, dust, moisture, etc., may accumulate between primary and secondary components. Since these materials change the dielectric characteristics between primary and secondary components with respect to air, the contaminants effectively reduce the gap between the components, thus facilitating arcing there between.
When hi-potential testing a board having a slot between the primary and secondary components, an arc will typically run along the surface of the board around the slot if the arc endpoints are near the edge of the slot. Extending the slot to increase the distance around it is one solution to force an arc to have to travel farther between endpoints, thus reducing the likelihood of arcing occurring. However, this may not be an option with PC boards where space is limited.
While the air-gap formed by the slots in the circuit board provides an increased breakdown voltage, electric equipment manufacturers and users are always looking for improved performance, reduced cost, ease of manufacturing and reliability of the devices. Thus, there is a need in the art for improved withstand voltage boundary performance, for example increased arc resistance, in electric devices. Further, there is a need for improving withstand voltage boundary performance without increasing manufacturing costs or complexity of the electric device.